The present invention is related to printing systems incorporating light emitting print bars as the imager, and, more particularly, to a print system using LED print bars whose operational temperatures are maintained within a given tolerance range.
Image print bars used in xerographic recording systems are well known in the art. The print bar generally includes a linear array of a plurality of discrete light emitting sources on a substrate. The print bar is optically coupled to a linear lens array. Light emitting diode (LED) print bars are preferred for many recording applications. In order to achieve high resolution, a large number of light emitting diodes, or pixels, are arranged in a print bar and means are included for providing a relative movement between the print bar and the photoreceptor so as to produce a scanning movement of the print bar over the surface of the photoreceptor. Thus, the photoreceptor may be exposed to provide a desired image one line at a time as the LED print bar and associated lens array is advanced relative to the photoreceptor either continuously or in stepping motion. Each LED pixel in the print bar is used to expose a corresponding area on the photoreceptor to a value determined by image defining video data information.
In a color xerographic system, a plurality of LED print bars may be positioned adjacent the photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth print bar may be added if black images are to be created as well.
FIG. 1 shows a prior art single pass color configuration having three print bars, 10, 12, 14. The print bars, each comprising an LED array and a coupling gradient index lens array (10A, 10B, 12A, 12B, 14A, 14B, respectively), are addressed by video image signals whose application is controlled by control circuit 15. Each print bar is optically coupled to focus the emitter outputs to form three spaced latent images l.sub.1, l.sub.2, l.sub.3 on the surface of photoreceptor belt 16. The optical coupling is accomplished by the gradient index lens arrays 10B, 12B, 14B, the lens array sold under the name SELFOC.sub..TM. a trademark of Nippon Sheet Glass Co., Ltd. Upstream of each exposure station, a charge device 18, 20, 22 places a predetermined charge on the surface of belt 16. Downstream from each exposure station, a development system 26, 28, 30, develops a latent image of the last exposure without disturbing previously developed images. Further details of xerographic stations in a multiple exposure single pass system are disclosed in U.S. Pat. No. 4,660,059 whose contents are hereby incorporated by reference.
With such a system as that disclosed in FIG. 1, each colored image must be precisely aligned such that all corresponding pixels in the image areas are registered. The print bar alignment requirements are that pixels of each bar be aligned in the scan or Y-direction of FIG. 1 so that each active write length is equal. The print bar must also be aligned in the skew or X-direction. This alignment must be maintained through continous revolutions (passes) of the photoreceptor.
To maintain exact color registration of each image, typically to a tolerance of .+-.0.1.mu., the overall length of the write area, the pixel to pixel placement, and the straightness of the image line must all be within the required exacting tolerance. One of the most difficult manufacturing tolerances to achieve is the overall or active write length of an image print bar. For example, for a 14.33" LED print bar with 300 spi resolution, 4299 pixels are aligned in the active write area and a .+-.15.mu. tolerance in write length is typical.
A specific problem in correcting exact image-to-image registration, and one which is addressed by the present invention, is the change in length an LED print bar undergoes when subjected to temperature increases (thermal expansion), which are caused either by heat generated internally to the print bar, or by heat absorbed by the print bar from the surrounding machine environment.
Typically, accurate LED print bars are formed on a single ceramic substrate with a CET (coefficient of thermal expansion) on the order of 7.0.times.10.sup.-6 linear units /.degree.C. To achieve proper registration (for a .+-.10.mu. tolerance due to thermal effects) of all pixels over a 364 mm write zone (B4 paper size), the temperature of all multiple print bars would have to be held to .+-.3.9.degree. C. An additional factor which must be considered is the need to compensate for the decrease in conversion efficiency of electrical to optical energy. For example, GaAsP LED material illumination efficiency decreases approximately 0.8% per .degree.C.
According to the principles of the present invention, a thermal controller system is provided which maintains multiple print bar temperatures within specific required limits. The temperature of each print bar is sensed and representative signals sent to a machine thermal controller to provde individual heating or cooling to maintain the print bar temperatures within a given tolerance as required for a dot-to-dot placement accuracy. More particularly, the invention is directed towards a thermal control system for maintaining the relative temperature of multiple print bars within a specified temperature differential range comprising:
a heater connected to each print bar and adapted, when energized, to increase the print bar temperature, PA1 a temperature sensor associated with each print bar adapted to continually sense the operating temperature of the associated print bar and to generate an output signal representative thereof, PA1 a cooling mechanism operatively coupled to each print bar to provide a cooling medium to one end of the print bar, and adapted, when energized, to decrease the temperature of the associated print bar, PA1 system control means for selectively controlling the temperatures of each of the print bars, said temperatures represented by signals from the associated temperatures sensor, and for detecting a predetermined temperature differential between two or more print bars, said control means further adapted to control the operation of said heaters and cooling mechanisms to restore the sensed temperature differential to within the specified differential range.
The following references have been identified in a prior art search:
U.S. Pat. No. 4,865,123 to Kawashima et al. discloses an apparatus for circulating a cooling fluid through a plurality of cooling modules for cooling electronic components. The apparatus includes a plurality of supply lines arranged independently and in parallel to each other. Each of the supply lines supplies coolant to an individual cooling module. At one end, the supply lines draw coolant from a mixing tank having a relatively large volume, and at the opposite end, the supply lines return the coolant to the mixing tank, wherein the coolant is circulated so that its temperature is kept uniform throughout. Each supply line includes a pair of pumps 3, check valves 4, and a heat exchanger 5.
U.S. Pat. No. 4,601,328 to Tasaka et al. discloses a method for temperature balancing control of a plurality of heat exchangers used in parallel. The temperatures of a medium flowing through the parallel heat exchangers are sensed at the same position in each of the plurality of heat exchangers, and the sensed temperature values are respectively compared with a temperature setting value, so as to calculate control signals for balancing the temperatures of the medium flowing out of the heat exchangers. Regulation means for each of the respective heat exchangers are responsive to the control signals to effect temperature balance of the medium.
In addition, co-pending application Ser. No. 07/773,793, filed on Oct. 9, 1991, and assigned to the same assignee as the present invention, discloses a method and apparatus for maintaining print bars at the same temperature by circulating a cooling medium through each print bar assembly.